Trees in silvopastoral systems can accumulate carbon (C) and nutrients under their canopies. Most studies measure only net changes in organic matter and nutrients without evaluating turnover of soil organic matter. Here, the change in vegetation cover from caatinga, a semideciduous thorn forest (principally C₃ metabolism) to buffel grass (Cenchrus ciliaris L.) pasture (C₄ metabolism) was used to quantify in situ input and turnover rates of organic carbon 14 yr after land-use changes. The accretion of C under new pasture and loss of original caatinga C was studied for whole soil (WS) and light fraction (LF). The effects of two tree species preserved during selective clearing and one species planted after complete clearing of caatinga were evaluated. All trees prevented organic matter mineralization that occurred in surrounding cleared pasture. The C mineralization under pasture was twice as high (66% loss) in LF as in WS (34% loss) over 14 yr. The C₄–C was similar under and outside the remnant and planted tree canopies, i.e., the input of new C₄–C did not compensate for the loss of old C₃–C that occurred following caatinga clearing and pasture establishment. The organic matter in this tropical, semiarid region mineralized rapidly with C half lives between 9 and 16 yr for LF and between 11 and 28 yr for WS. The ¹³C data indicate that elevated C contents under preserved (WS and LF) and planted (LF) trees, relative to the pasture outside the tree canopies, largely represent C₃–C inherited from the caatinga. In this silvopastoral system, derived from land-use changes from dry forest, the islands of fertility and organic matter under the trees were not built up and represent preserved, rather than new, C inputs.